Apparatuses and methods for treating fluids with ultrasound are generally known. Frequently, the fluids are to be mixed together to produce, for example, emulsions or dispersions, to cause precipitation reactions or to perform sonochemical reactions. In conventional apparatuses and methods, two different fluids are mixed together for this purpose and then treated by ultrasound. Due to rapidly occurring reactions between the two fluids, the subsequent ultrasonic treatment of the fluid mixture is frequently unable to produce the desired mixing or reaction effect, since the ultrasound treatment starts too late.